1. Field of the Invention
This invention relates to a method for capturing and retrieving objects on the high seas. The described invention is particularly useful in the retrieval of anchors used in positioning offshore construction and drilling vessels.
2. The Prior Art
In the art practiced today the requirements for very accurate and steady positioning and for precision movements of construction and drilling vessels are commonly achieved by tethering the vessel within an array of anchors with each anchor line attached to its own independently operable winch onboard the vessel. By coordinated operation of the winches, the vessel can be held in any fixed position or moved in any desired direction within the area defined by the outlying anchors and with great precision. Before the vessel can move to a position outside the area defined by the anchor array, the anchors must be moved so that the new array encompasses the desired new location. It is common for a vessel to have an eight to twelve anchor array and for pipe laying barges it is common for each anchor to be moved several times a day. Because anchors are almost always placed at considerable distances from the vessel, anchors are retrieved for storage or relocation by supporting vessels (tugs).
Each anchor has attached to it a pendant line heavy enough to raise the anchor, the free end of which is floated on the surface by an anchor buoy which must be large enough to support the great weight of the pendant line. Actually, the pendant line is usually free to slide through a hole in the buoy and is prevented from falling by a loop formed on the free end of the pendant line. To retrieve an anchor, the support tug must locate and draw alongside the anchor buoy, attach a winch line to the pendant line loop and raise the anchor by winching in the winch line and the pendant line. The pendant line is drawn through the anchor buoy during this operation pinning the buoy against the tug. The anchor is raised only to the bottom of the anchor buoy and, thus, remains underwater. The anchor, though now raised, is still attached to the construction vessel via the anchor line. By coordinated action of the tug and the anchor line winch onboard the construction vessel, the anchor can be relocated anywhere within a radius limited only by the total length of the anchor line, or the anchor may be brought back to the construction vessel for storage. Typical component sizes are 4,000 to 5,000 feet of anchor line of 2 5/8" diameter wire rope weighing nearly one ton per 100 feet, 100 feet to more than 1,000 feet of anchor pendant line of 2" to 2 5/8" wire rope weighing from 1/2 ton to nearly one ton per 100 feet, 15 to 30 ton anchors, 8 to 10 ton anchor buoys measuring 8 feet in diameter by 15 feet long. Components of these magnitudes are very difficult to handle.
The most difficult part of the operation is attaching the winch line to the pendant line loop, which must now be performed manually. While the tug captain attempts to keep his tug in a relatively fixed position along side the anchor buoy, workmen standing on the tug must reach over the side a minimum of four feet to manually attach the heavy winch line to the anchor pendant line loop. The working position is awkward and dangerous even in calm seas when the rolling and pitching of a tug can make a workman's footing extremely precarious. In only moderate (Beaufort force 4) seas, differences between the tug and the anchor buoy in mass, shape and exact position on the waves causes relative asynchronous motion between the two with the result that they often collide with great force. The higher the seas the more difficult and dangerous the operation. It is widely recognized that anchor retrieval cannot be carried out in greater than force 5 seas and that this is because it is too difficult and dangerous under such conditions to attach a winch line to the anchor pendant line. Until recently this limitation was not a significant factor because construction vessels could not work in seas higher than about force 5. However, with the recent development of more stable construction vessels, the problems of anchor retrieval have become a limiting factor and a number of attempts have been made to solve the problem.
One method uses a remote-releasable hook suspended on a service line from a support vessel as disclosed in U.S. Pat. No. 3,927,636 issued Dec. 23, 1975 to Childers, et al. The support vessel sails so as to cause the hook to snag the anchor line underwater and near the construction vessel and then causes the hook to slide down the anchor line to the anchor by sailing toward and past the anchor. The anchor is then raised by raising the hook. U.S. Pat. No. 3,929,087 issued Dec. 30, 1975 to Montgomery appears to be essentially the same but without the catch device or the remote-release feature. It is known that in practice the snagging operation is difficult. The deployment of anchors using a special remote-release hook is described along with another configuration of the remote-release catch hook in U.S. Pat. No. 3,931,782 issued Jan. 13, 1976 to Childers, et al. It is also known that at least one company operating in the North Sea attempted anchor retrieval by placing a large ring around the anchor line. A tug with a line attached was used to retrieve the anchor. The method was abandoned as impractical because of the difficulty of keeping enough tension on the ring line to prevent the anchor from sliding back to the bottom of the sea.
A retractable basket is disclosed in U.S. Pat. No. 3,993,011 issued Nov. 23, 1976 to Garland for recovering a special anchor buoy, which requires that the support vessel be fitted with the large and complex special handling system.
Another attempted solution required the use of a catamaran tug which passed over the anchor buoy so that the buoy was caught in a cradle suspended between the two hulls of the tug. The system was found to be impractical in high seas due to the damage caused by violent impact between the tug and the anchor buoy.
Another proposed solution would have the heavy anchor pendant line remain on the sea bottom and utilize a small, light buoy and line to mark and raise the anchor pendant line for retrieval, as disclosed in U.S. Pat. No. 4,051,800, issued Oct. 4, 1977 to Farstad.
The use of a large electromagnet to capture the anchor buoy long enough to secure a winch line to the anchor pendant line loop has also been proposed. A full scale electromagnet has been built and tested on land but has not been subjected to sea trials.